A subpopulation of germinal center B cells differentiate directly into antibody forming cells upon secondary immunization.

In the present study we sought to test the hypothesis that GC B cells may be stimulated to differentiate into AFCs during the induction of the secondary antibody response. To test this we gave HRP immune mice a booster immunization and looked for AFCs in the GCs making anti-HRP specific antibody. Within 3 days cells in the GCs were making anti-HRP. Most of the cells were just starting to make immunoglobulin in the rough endoplasmic reticulum but some were nearly mature plasma cells like those in the medullary cords. Furthermore, we isolated GC B cells after a boost with OVA and tested their ability to differentiate directly into AFCs. Data obtained using an OVA specific RIA indicated that purified GC B cells would differentiate and produce specific antibody in the absence of added T cells or antigen. Examination of the cells after 5 days in culture indicated that many of them had differentiated into large plasma cells. These results obtained both in vivo and in vitro demonstrate that shortly after booster immunization GC B cells receive the appropriate antigen and T cell signals in vivo to differentiate and become AFCs.